The present invention relates to the improvement of binders for the formation of a coat layer, which are used in the method for the manufacture of molds where a refractory coat layer is formed on a mold pattern (hereinafter referred to as "pattern") and thereafter the pattern is taken off and the refractory layer is fired to obtain a desired mold.
In general, the refractory coat layer is formed on a pattern as an accumulative structure by repeating a procedure comprising coating said pattern with a liquid binder or a mixture slurry comprising the liquid binder and a refractory powder, sanding, if desired, drying and hardening the thus obtained coated layer, until a desired thickness is obtained for an accumulative layer.
One of the remarkable improvements in the precision casting technique where the molds as obtained by the formation of the said refractory coat layer on the pattern are used was the improvement of binders for molds, and the other was the improvement of pattern materials.
The improvement of the pattern materials has been developed mainly for the purpose of the improvement of the moldability, dimension stability and strangth or of the reduction of the cost; and for example, blends of various kinds of waxes or waxes as improved by the addition of a flux as well as improved materials of naphthalens, plastics or ureas have heretofore been proposed. In particular, urea-type materials are water-soluble and have both high strength and high dimension stability and furter are inexpensive, and therefore, a lot of these materials have been used as these are almost satisfactory materials, although some careful attention is to be paid to the moisture in air in the treatment of the materials.
On the other hand, numerous kinds of proposals for the improvement of the binders have heretofore been made, but no sufficient improvement has been attained as yet, up to the present. In the initial stage, aqueous silicasols, alcohol silicasols or ethyl silicates have been used. However, when kept in contact with water-soluble patterns, the aqueous silicasols corrode the surface of the patterns. Regarding the alcohol silicasols, any high strength cannot be imparted to the green molds before fired, and the molds are apt to be broken in the operation for the removal of patterns. The ethyl silicates cannot form fired molds of high strength. In addition, the binders obtained by the hydrolysis of the ethyl silicates are poorly stable, and therefore, it is difficult to manufacture molds of a certain quality from these binders.
Japanese Patent Publication No. 32482/73 has proposed a binder which contains a colloidal silica and a hydrolyzed alkyl silicate and which is free from water, by blending an alkyl silicate and an appropriate amount of an aqueous silicasol. This binder, however, is not so stable as being able to be preserved for a long period of time, and further, when kept in contact with the surface of a pattern made of a water-soluble material, this corrodes the surface of the pattern.
Japanese Patent Publication No. 40366/79 has proposed a binder with an improved stability, which comprises a colloidal silicasol, a hydrolyzed alkyl silicate and a glycolether as blended in an appropriate proportion. However, this binder is also unfavorable, since this dissolves the surface of the pattern made of a water-soluble material, when kept in contact therewith.
Japanese Patent Publication No. 22929/79 has proposed still another improved binder, which comprises 30-60% of a nethyl silicate mixture comprising various polyethyl silicates, 20-50% of an anhydrous solvent having a dipole moment of 1 Debye or less and 8-30% of an amino group-containing organic functional hydrophilic silicon compound. However, the binder of this kind still is insufficient in view of the fact that a higher precision is desired at present in the field of the precision casting technique. This is especially because, when a mold manufactured by the use of this binder is used for casting, the surface of the casting is made rough. Further, in the step for the formatio of the green mold, interlaminar cleavage is apt to occur in the accumulative coat layer, and if the mold with such interlaminer cleavage is used in casting, the mold is often broken when a molten metal is put thereinto. Thus, the use of this binder causes various unfavorable results.
The difficulty in the improvement of binders for the manufacture of precision casting molds results from the difficulty in the discovery of such binders as being able to satisfy all the subject matters that no problem occur in the step of coating a binder slurry on a water-soluble pattern of high strength and high dimension accuracy, the step of drying the coat layer, the step of removing the pattern from the coat layer, the step of firing the coat layer and other various steps, that the binder itself has a sufficient stability and thus molds of a certain characteristic may easily be obtained with the binder and that the molds after fired have no defects.